The present invention relates to ink used in recording instruments and more particularly to non-splattering inks used in oscillographic type pen recorders and the like.
An inking system and ink therefore typical of the environment as wherein the present invention is applicable is set forth in the U.S. Pat. No. 3,906,513 to Siegelman et al. which is assigned to the common assignee of this application. In such apparatus, an oscillograph pen, comprising a metallic capillary tube connected to a supply of ink, is biased against a paper recording medium which is moved slowly and continuously under the pen tip. As the paper is thus moved under the pen tip, an electrical signal is applied to the oscillographic pen motor which drives the capillary tube type pen from side to side thus producing a permanent marking of the electrical waveform being applied at the input to the oscillograph. Such apparatus is prone to a number of problems. As the pen terminates its excursion in one direction toward the side of the recording paper and reverses direction to move in the opposite direction, the ink contained therein has a tendency to continue in the first direction as a result of the force of inertia on the ink. Additionally, as the pen tip is traversing the paper at a high rate of speed in response to a rapidly changing electrical signal of high frequency and amplitude, there is a tendency for the ink to discontinue marking on the surface of the paper for periods of time. The former problem is referred to as "splattering" and the latter as "skipping". Considerable research has been conducted by manufacturers of such oscillographic type pen recorders and their suppliers to find a workable solution to both the splattering and skipping problems. Unfortunately, the solutions often tend to be mutually exclusive. That is, a highly viscous ink would not tend to splatter but, on the other hand, would be highly prone to skipping. By contrast, a low viscosity ink that would not skip would have a high tendency to splatter.
In the aforementioned patent to Siegelman et al., as well as in the patents to Forsyth, Jr. (U.S. Pat. No. 3,477,862) and Packer (U.S. Pat. No. 3,692,548), a high molecular weight (about 500,000 to 5,000,000) polyethylene oxide is incorporated into the ink. The polyethylene oxide is one of certain polymers which, when in solution, exhibit the property of filament formation, or the tendency to pull out into long threadlike structures, as when a stirring rod is withdrawn from a container of the solution, for example. This ability to form filaments is believed to be a manifestation of the high elongational viscosity of polymer solutions. The tendency to form filaments is known as pituity. One way that pituity is measured is via a drop technique whereby a fixed quantity of solution is ejected from a microliter syringe. The time from when the drop of solution falls to a watch glass at a fixed distance below the syringe to when the thread of solution trailing from the watch glass to the top of the syringe breaks is taken as a measure of pituity.
The property of filament formation in an ink used in an oscillographic type recording system has two benefits. First, there is a tendency to reduce drag. This is because of the tendency of each particle of ink to pull the next adjacent particle through the capillary and for that ink on the surface to draw the next adjacent particle from the pen tip. The result is a tendency not to skip. At the same time, the filament formation tendency tends to prevent the disassociation of adjacent particles. It is the disassociation of adjacent particles which causes splattering. Thus, the inclusion of the polymer having the property of filament formation in the ink tends to solve both the skipping and splattering problems heretofore identified.
While the inclusion of high molecular weight polyethylene oxide into the ink has bestowed the desired benefits of non-splattering and nonskipping desired, it is not without attendant drawbacks. Under normal circumstances, other additives to the ink, including the dye thereof, have a tendency to oxidize the polyethylene oxide thus destroying its long chain molecules and attendant filament forming qualities. This is particularly noticeable with the sulfone groups of an acid dye. The presently known formulations of non-splatter ink incorporating polyethylene oxide require dyes with amine, (-NH.sub.2.sup.+) or dimethyl amine, [-N(CH.sub.3).sub.2.sup.+ ], groups or basic dyes such as basic violet 2, basic violet 3, or basic green 4, in order not to oxidize the polyethylene oxide. The basic dyes do not exhibit the tendency to rapidly oxide the polyethylene oxides as do the acid dyes. On the other hand, the basic dyes stain the users hands and the instrument badly while the acid dyes are water soluble and strain very little. Thus, to the user, ink incorporating acid dyes are preferred for cleanliness. Additionally, it is often desirable to have a single dye component black ink which is available only as an acid dye.
With the use of a pituity agent that is easily oxidizable, a washable ink that is stable at room temperature for more than approximately two months is not possible. The ink can be refrigerated to stop the deterioration process, but such refrigeration facilities are often not available at the sites wherein the recorders are desired to be incorporated.
Thus, it is the object of the present invention to provide a washable non-splatter ink incorporating a pituity agent which is not prone to oxidation when used in conjunction with acid dyes.